Device, method and system for determining the road surface condition

ABSTRACT

The invention relates to a device, a method and a system for determining a road surface condition, where the surface condition is one of dry, wet or icy. The device comprises a reflectance spectrometer which senses the reflectance properties of the road at one or several wavelengths and uses these reflectance properties to determine the surface condition. The reflectance spectrometer is a wavelength modulation spectrometer, preferably for the near infrared region. The system determines the surface condition and indicates it to a user of the system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication PCT/SE2003/001570, filed Oct. 9, 2003, designating theUnited States of America, which claims the benefit of Swedish PatentApplication No. SE-0202987-C2, filed Oct. 10, 2002.

The present invention relates to a system, a device and a method fordetermining the presence or absence of liquid water or ice according tothe introductory portions of the independent claims. In particular, itrelates to such a system, device and method using wavelength modulationspectroscopy.

BACKGROUND OF THE INVENTION

A number of solutions for solving the problem of determining possibleslipperiness of road surfaces, in particular aimed at detecting presenceor absence of water or ice on the road surface, are known. Older methodsfor determining the road surface condition using mechanical arrangementsare known but are prone to faults and wear. A number of solutions to theproblem of detecting water or ice on a road surface at a fixed positionhave also been suggested, but are generally not applicable to theproblem of determining the road surface condition at or near a movingvehicle. For that problem, remote sensing methods using spectroscopicalmethods have become the dominating solution, and in particular nearinfrared spectroscopical methods due to the distinct spectroscopicalproperties of liquid and frozen water in that wavelength interval.

One of the earliest patents relating to this subject is U.S. Pat. No.4,271,091 (1981) wherein a method of detecting ice on road surfaces bydetecting an amplitude modulated light beam in the infrared regionreflected from the road surface is disclosed. The method suffers fromthe drawback that there is no provision for separating reflectancechanges occurring due to presence of ice from those occurring due topresence of water on the road surface, or due to changes in thereflectance properties of the asphalt or concrete constituting the roadpaving.

U.S. Pat. No. 5,218,206 (1990) discloses a method of detecting ice orwater on road surfaces by detecting the reflectance of the road surfaceat two separate wavelengths in the infrared region. The methodcalculates the ratio between the two reflectances, and indicates thepresence of water if the ratio exceeds a certain level or the presenceof ice if the ratio falls below a certain level. If the ratio remainswithin an intermediate level, the method indicates that the road surfaceis dry, but unfortunately the ratio may also fall within thisintermediate range if certain proportions of water and ice are presentat the road surface.

Assuming the reflectance to be influenced by three parameters only; roadpaving reflectivity, effective liquid water layer thickness andeffective ice layer thickness, three independent parameters need to bemeasured, and a number of solutions using three or more wavelengths havebeen suggested, i.a. U.S. Pat. No. 5,962,853, proposing detection at atleast four wavelengths. Unfortunately, the absorption of light in nonturbid media adheres to the Beer-Lambert law, stating that thetransmission through the medium decreases exponentially with increasinglayer thickness. For a detection system with finite signal dynamics,this corresponds to a very limited dynamic range in terms of layerthickness variations. To solve this problem, one may detect the presenceof water or ice using several different wavelength intervals havingdifferent absorption coefficients, where detection in each intervalgives a reliable indication of water or ice presence for a range ofsubstance thicknesses. Combining results from measurements in severalsuch intervals, an acceptable total layer thickness tolerance isachieved. Unfortunately, this implies detecting reflectances at acomparatively large number of wavelengths, necessitating a complex, andtherefore expensive arrangement.

Wavelength modulation spectroscopy is a particular form of spectroscopywhere the used light wavelength is modulated at a frequency f. Afterinterfering with a substance, usually a gas, the wavelength modulationgives rise to amplitude modulation at frequencies being multiples of thewavelength modulation frequency f, and the amplitude modulated signal atone of these multiples of f is used for detection. With wavelengthmodulation spectroscopy it is possible to achieve higher signal to noiseratio than with other corresponding spectroscopical methods, thus makingit possible to detect and measure substances in smaller concentrationsthan otherwise. In U.S. Pat. No. 6,356,350 a form of wavelengthmodulation spectroscopy is disclosed, where signals are detected at morethan one of these multiples of f concurrently, and the signals receivedare used to calculate properties of a gas being measured, such as itsconcentration, temperature or pressure. The document does however notdisclose a method of concurrently measuring the amount of two or moresubstances with overlapping spectral features, using a single detectedmodulated wavelength. Neither does the document disclose a method ofderiving information on whether the substance being detected is turbidor not.

An object of the invention is therefore to provide a system, a methodand a device which overcome the above mentioned problems with prior artsurface condition detection devices.

These and other objects are attained by a system, a method and a deviceaccording to the characterising portions of the independent claims.

SUMMARY OF THE INVENTION

The invention relates to a device for determining a road surfacecondition, where the surface condition is one of dry, wet or icy. Thedevice comprises a reflectance spectrometer which senses the reflectanceproperties of the road at one or several wavelength and uses thesereflectance properties to determine the surface condition. Thereflectance spectrometer is a wavelength modulation spectrometer,preferably for the near infrared region. The device may wavelengthmodulate the light being reflected by the surface before or afterhitting the surface. The device uses light at a chosen wavelenght, beingwavelength modulated at a frequency f, and detects the resultingamplitude modulation of the light at more than one multiple of f, usingthe amplitude of the amplitude modulations at different multiples of ffor determining the road surface condition.

The device may sense the reflectance properties of a surface at morethan one wavelength and use the additional information on thereflectance properties to determine the structural properties of thedetected liquid water or ice, i.e. determining whether the water or iceis clear or turbid. This information about the structural properties maybe used to assess the slipperiness of the surface.

The invention further relates to a method for determining a road surfacecondition using wavelength modulation spectroscopy, and yet further to asystem for determining and indicating a surface condition to a user ofthe system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of the ice and water detection device.

FIG. 2 shows a second embodiment of the ice and water detection device.

FIG. 3 shows an embodiment of a chopper wheel usable in the first andsecond embodiments.

FIG. 4 shows theoretical signal values received for varying layerthicknesses.

FIG. 5 shows how different signal value combinations are used to assessdifferent surface conditions.

FIG. 6 shows theoretical signal values at different wavelengths.

FIG. 7 shows theoretical signal values taking imperfections into account

FIG. 8 shows a third embodiment of the ice and water detection device.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of the ice and water detection devicewhich uses prisms as dispersing elements. The embodiment comprises alight beam emitter with suitable optical properties, constituted by alight source 1, and a first focusing element 3 focusing a portion of thelight emitted by the light source on an aperture 5. The light source 1is schematically shown as an incandescent lamp, and the first focusingelement 3 is drawn as a pair of planoconvex lenses, but this is chosenonly for illustrating the fundamental function of the device.

The diverging light beam emitted from the aperture 5 is then transmittedtowards a first wavelength selective system. In the wavelength selectivesystem the beam is collimated by a first lens 7, and the collimated beamis directed through a first dispersing prism 9. The light beamtransmitted through the prism is dispersed into a range of wavelengths,which are focused by a second lens 11 onto a selection element 13 whichonly transmits selected segments of the light focused onto it. Theselection element 13 is here embodied as a chopper wheel 24, shown inFIG. 3. The primary function of the chopper wheel 24 is to transmitselected portions of the light of the continuous range of wavelengthsfocused onto it, through three non-circular apertures 26, 28, 30. As thechopper wheel 24 is rotated, the portion of the apertures 26, 28, 30exposed to the light focused onto it shifts, as indicated by the arrowin the drawing, thereby selecting a changing set of wavelengths beingtransmitted through the chopper wheel 24. Three diverging light beamstransmitted through the chopper wheel 24 are again focused by a thirdlens 15, and the collimated beams enter a second dispersing prism 17.Using a second dispersing prism 17 with dispersing properties identicalto that of the first dispersing prism 9, the three collimated beamsemerge from the second dispersing prism 17 overlapping each other andbeing parallel.

The beam emitted from the second dispersing prism 17 is partiallytransmitted through a beam splitter 19, and hits the road surface. Lightreflected from the road surface hitting the beam splitter 19, ispartially reflected by the beam splitter 19 and transmitted in adirection orthogonal to that of the outgoing beam. The reflected beam isthen focused by a fourth lens 21 onto a detector 23, detecting thesignal from the road surface.

The detector could for example be an InGaAs, Ge, InAs, PbS or apyroelectric detector. The advantage of pyroelectric detectors, ascompared to the others, is the lower cost and its flat spectralresponse, but it does have a detectivity two to four orders of magnitudelower than the other detector types. The total light throughput of thesystem is related to the dispersive power of the dispersive element,i.e. the prisms shown in the embodiment above. Even with prisms made ofsubstances which are highly dispersive in the wavelength range ofinterest, such as Si or one of the Irtran glasses, the light throughputmay be insufficient for using low detectivity detectors.

FIG. 2 shows a second embodiment of the detection device which insteadof prisms uses reflecting gratings, which have a much higher dispersivepower, as dispersing elements. The embodiment comprises a light beamemitter identical to the one in FIG. 1, and the emitted light is focusedby a second focusing element 8 drawn as a pair of planoconvex lenses.The focused beam passes, at its focal point, above a first mirror 2(being positioned in a direction below the paper plane of the figure),and is then directed to a second wavelength selective system.

In the second wavelength selective system the beam is collimated by afirst lens 7, and the collimated beam i directed towards a reflectivegrating 10. The light beam reflected from the grating is dispersed intoa range of wavelengths, which are focused by a fifth lens 12 onto aselection element 13 which only transmits selected segments of the lightfocused onto it. The selection element 13 is here embodied as a chopperwheel 24, shown in FIG. 3. The three light beams transmitted through thechopper wheel 24 are reflected back through the chopper wheel 24 by asecond mirror 6, slightly tilted downwards (in a direction out of thepaper plane of the figure), are recollimated by the fourth lens 12, andare reflected back from the grating 10 overlapping each other and beingparallel. The three overlapping beams are then focused by the first lens7 onto the first mirror 2, which reflects the beams towards a sixth lens4.

The sixth lens 4 collimates the beams and directs them to a set-upcomprising a beam splitter 19, a fourth lens 21 and a detector 23,identical to the one in the first embodiment.

Obviously, the embodiment could alternatively have been arranged with atransmission grating, while a set-up similar to the first embodiment,having two gratings, would be unnecessary due to the potentially highdispersive power of the gratings, and inconvenient due to the cost ofgratings. The higher optical throughput of the system does on the otherhand makes it possible to use cheaper detectors with lower detectivity.

In this embodiment the wavelength selection element 13 couldalternatively have been embodied as a set of tuning fork type opticalchoppers, which are essentially mirrors mounted on the ends ofelectromechanically driven tuning forks. The fork may resonate at ahigher frequency than a rotating disc type chopper device, and may, ifdriven at its resonance frequency, be very insensitive to disturbances.This kind of chopper also has a longer life span, but may be moreexpensive.

FIG. 3 shows an embodiment of a chopper wheel 24 usable in the first andsecond embodiments. The solid areas on the chopper wheel 24 indicateapertures in the otherwise non transparent chopper wheel 24. A portionof the non circular rings 26, 28, 30 indicated by the area 31 is what isillustrated in cross section in FIGS. 2 and 3 as the wavelengthselection element 13. As the chopper wheel 24 rotates, the distance fromthe apertures to the center axis 29 of the chopper wheel 24 will shiftback and forth periodically, with different periodicity for thedifferent non circular rings 26, 28 and 30. The innermost non circularring 30 moves back and forth three times per rotation of the wheel 24,the next ring 28 four times and the outermost non circular ring 26 fivetimes per rotation.

The non circular rings 26, 28, 30 will thus select light beams atseparate wavelengths, and as the wheel 24 rotates, the first wavelengthselective system will emit a beam of light of three differentwavelengths, each wavelength modulated at three, four and five times therotational frequency of the wheel 24.

Any constant intensity wavelength modulated light beam experiencingwavelength dependent absorption will become amplitude modulated atfrequencies corresponding to multiples of the wavelength modulationfrequency. The DC signal will be proportional to the reflectance itself,i.e. the zeroeth derivative of the absorption with respect to thewavelength, the size of the amplitude modulation at the wavelengthmodulation frequency will be proportional to the derivative of theabsorption with respect to the wavelength, and the size of the amplitudemodulation at twice the wavelength modulation frequency will beproportional to the second derivative of the absorption with respect tothe wavelength etc.

As water and ice have absorptions with different wavelengthdependencies, a wavelength modulated light beam being transmitted thoughwater or ice will become amplitude modulated in different ways, givingrise to different sets of amplitudes of the degree of amplitudemodulation at different multiples of the wavelength modulationfrequency. Assuming the wavelength dependence of the reflectance of thepaving to be small or zero, ie. it has a flat absorption curve as afunction of the wavelength, this will only give rise to a DC signal atthe detector which may be neglected. Denoting the amplitude of theamplitude modulation at the frequency corresponding to the wavelengthmodulation frequency as S₁ and the amplitude of the amplitude modulationat twice the frequency corresponding to the wavelength modulationfrequency as S₂, the relation between these amplitudes may be discussedusing a diagrammatic approach.

Plotting S₁ and S₂ on the x- and y-axes of the graph in FIG. 4,respectively, for different ice (solid line) and water (dashed line)layer thicknesses at an arbitrarily chosen wavelength, curves similar tothe ones shown in FIG. 4 may be found. For any substance, both S₁ and S₂are obviously zero for a substance thickness of zero, and as thesubstance thickness increases, the curve deviates from the origin asindicated by the arrows on the curves. Eventually the substancethickness gets so large that the transmission through the substanceapproaches zero, and both curves then return to the origin. For anarbitrarily chosen wavelength, the proportions between S₁ and S₂ are notfixed, so the curves are loop-like. This may make it difficult toseparate signals arising from presence of water from those arising frompresence of ice, and if the curves cross it is for certain thicknessesnot possible to separate them at all.

By choosing wavelength for detection properly, the proportions of S₁ andS₂ for both curves remain nearly fixed for any layer thickness, and theloops look nearly like straight lines extending in different directionsfrom the origin of the graph, as in FIG. 5. The figure also shows howdifferent parameter area sectors are interpreted as different surfaceconditions. An area DRY extending a small distance from the origin isinterpreted as dry road surface, and two sectors ICE and WET extendingalong and including the loops corresponding to the ice signal loop andthe water signal loop, are interpreted as purely icy and purely wet roadsurface, respectively. An area MIX extending between these last twoareas is interpreted as a road surface covered by a mix of water andice. The parameter area sectors outside these four areas may be usede.g. for fault tracing.

The radius of the circular area DRY within which the parameter values S₁and S₂ are interpreted as indicating a dry surface, is defined by thenoise level of the signal. The noise is caused by varying backgroundreflection due to the graininess of the road surface, electronic noiseand other factors. As the noise in the S₁ and S₂-parameters may bedifferent and dependent, the DRY area might in practice be of any othershape but circular, the circular area chosen here is for simplicityonly.

The width of the WET and ICE parameter areas is partially set by noiseconsiderations, but also has to include factors such as temperatureaffecting the absorption curves for both water and ice, and salinityaffecting the absorption curve for water. Increased salinity in waterwill affect the absorption curve for water in a way similar to atemperature increase, which may be interpreted as an increase inapparent temperature. Apparent temperature changes in the ranges presentunder normal circumstances for ice or water changes the absorptioncurves slightly, which in the S₁-S₂ plane appears as slight angular andother shifts of the ice and water curves.

FIG. 6 shows ice and water parameter curves for two differentwavelengths, 33 and 34. Only two wavelengths are illustrated forsimplicity reasons only, even though the first and second embodimentsuse three wavelengths. Several different wavelengths are found in thenear infrared spectrum where the parameter curves are near linear, butfor different wavelengths the curves may have different angulardirections and extend different distances from the origin. Obviously,this needs to be compensated for, using different parameter area sectorsfor interpreting the road surface properties at different wavelengths.In the first and second embodiments of the inventions, the signals fromwhich S₁ and S₂ are derived, are modulated at different frequencies,making it easy to apply different surface property interpretation rules.If a set of wavelengths is found where the parameter curves overlap,different interpretation rules may not be necessary, and the modulationfrequencies at different wavelengths may be identical, simplifying thesignal processing.

Preferably, the set of wavelengths used are chosen such that in thepresence of clear water and/or ice a significant signal is received atat least one wavelength. This means that for the thinnest substancelayers of interest, a signal is received at the most sensitivewavelength, i.e. the wavelength at which the absorptivity is thelargest, while no signal is received at the other wavelength(s). As thesubstance layer thickness exceeds the interval where the most sensitivewavelength is active, i.e. where the substance appears completelyintransparent at that wavelength, a signal is received at the nextwavelength, while the substance still appears completely transparent atthe next wavelength etc. A set of wavelengths should therefore be chosensuch that any normally appearing clear substance layer thickness isdetectable.

If the substances are not clear, however, the Beer-Lambert law is notadhered, and may be replaced by the Kubelka-Munk equations. Under suchcircumstances, appearing e.g. in the presence of dirty water or ice,snow, frost or slurries of water/ice mixtures, significant signalcontributions may appear at several wavelengths simultaneously. This maybe used to derive information on the structural properties of thewater/ice layer on the road surface. From this information may beconcluded the slipperiness of the ice/water layer, which may bepresented to the user of the system according to the invention.

FIG. 7 indicates the result of a further imperfection of the arrangementon the ice and water parameter curves. In FIGS. 4-7 it is assumed thatthe wavelength modulation causes no residual inherent amplitudemodulation of the signal even in absence of water or ice, and theparameter curves thus starts and ends at the origin of the graphs. Ifsuch a residual amplitude modulation is present, the curves, here shownfor two different wavelengths 33 and 34, will originate at differentpositions in the S₁-S₂ plane. Again, the result of such flaws may becompensated for using proper signal processing.

FIG. 8 shows a third embodiment of the detection device which instead ofa dispersive element 9, 10, 17 and a wavelength selection element 13uses a pivoting dielectric filter 14. Here, the wavelength modulationoccurs after the light hitting the road surface has been received by thedetection device. To be able to separate light originating from thelight beam emitter of the detection device from background radiation,the aperture 5 of the light beam emitter is embodied as the aperture ofa chopper wheel for intensity modulation. The result is that amplitudemodulated light of a known frequency f_(A) is emitted from the lightbeam emitter and may be separated from background radiation. Theamplitude modulated light is then collimated by a sixth lens 4,partially transmitted through a beam splitter 19, and hits the roadsurface.

Light reflected from the road surface hitting the beam splitter 19, ispartially reflected by the beam splitter 19 and transmitted in adirection orthogonal to that of the outgoing beam. The beam is thentransmitted through a dielectric transmission filter 14, which isarranged at an angle slightly offset from the incoming beam. The filterangle is changed in a periodical manner, and the filter may for examplebe mounted on a galvanometer which periodically pivots the filter aroundan axis orthogonal to the beam direction, as indicated by the arrow inthe figure. The filter is arranged to transmit a set of suitablewavelengths, and as the filter is tilted, these wavelengths shift. Byvibrating the filter in a suitable way, the beam transmitted through thefilter will be amplitude modulated at frequencies related to thevibration frequency of the filter. Through proper signal processingdescribed below, the absorption properties of the road surface may bededuced. The beam is finally focused by a fourth lens 21 onto a detector23.

In this embodiment, the signal parameters of interest, S₁ and S₂, arenot found at the wavelength modulation frequency f_(λ), and twice thatfrequency 2f_(λ), but at f_(A)±f_(λ), and at f_(A)±2f_(λ). By selectingf_(A) and f_(λ), properly, f_(A)±f_(λ) and f_(A)±2f_(λ) may be detectedat conveniently low frequencies, allowing use of cheap, slow detectors.Meanwhile noise occurring as a result of the graininess of the roadsurface is picked up at f_(A), allowing choice of f_(A) at a low noisefrequency.

In this embodiment there is no direct way of separating signals atdifferent wavelengths by detecting them at different modulationfrequencies, as all are wavelength modulated at the same frequencyf_(λ). This implies that situations like the ones illustrated in FIGS. 6and 7 may be difficult to handle. Neither is it possible to to deriveinformation on the structural properties of the water/ice layer on theroad surface using the methods described above. All embodiments shownshould however be interpreted as illustrative only, and not as limiting.

In the examples presented above, the surface conditions are concluded bydetecting the reflectance properties at two or three wavelengths, butobviously any number of wavelengths may be used. Further, only thesignals S₁ and S₂ are discussed, but obviously S₀ and S₃, S₄ . . . etc.may be used to support the surface property identification algorithms.

The three embodiments shown have a light beam emitter 1, 3, 5 and awavelength selective system, where the latter acts to select suitablychosen wavelengths and wavelength modulate these before or after thebeam is reflected by the road surface. The light beam emitter may use anincandescent lamp, an LED or, if sufficient background light is present,may be eliminated altogether. Wavelength selective systems using prisms,gratings or dielectric filter are shown, but other solutions arepossible such as acousto optic modulators, which may have a much highermodulation frequency than any mechanical solution. Alternatively, thelight beam emitter and the wavelength selective system may be integratedinto a single functional unit using a wavelength modulated laser source.

The detection device may be mounted in a vehicle such that it may detectice or water under the vehicle, but may alternatively be forwardlooking, giving the driver an advance warning of upcoming wet or icysections of the road. For such a forward looking detection devicefurther functionality may be integrated into the system, for example asystem which makes the detection device track the road in front of thevehicle as the road turns, or have two or several detection areas, suchas one nearer and another further from the front of the vehicle.

The detection device is intended to be part of a system for determiningthe road surface condition including a road surface indicator,preferably mounted in the vehicle compartment. The road surfaceindicator shows the present road surface condition and may warn atsudden changes in road surface conditions.

Although the invention has been described in conjunction with a numberof preferred embodiments, it is to be understood that variousmodifications may still be made without departing from the scope of theinvention as defined by the appended claims. One such modification is touse the invention for determining the surface condition of objects otherthan roads.

1. A device for determining a surface condition, said device comprisinga reflectance spectrometer which is arranged to sense the reflectanceproperties of a surface at at least one wavelength and using saidreflectance properties to determine the presence of at least one ofliquid water or ice, characterised in that said reflectance spectrometeris a wavelength modulation spectrometer which modulates the wavelengthof light at a frequency f, said wavelength modulation spectrometer beingprovided with means to detect the resulting amplitude modulation at morethan one multiple of said frequency f.
 2. A device for determining asurface condition according to claim 1, characterised in that saidwavelength modulation spectrometer comprises a wavelength selectivesystem arranged to select and wavelength modulate light of at least onewavelength, where said wavelength selective system comprises at leastone of a chopper wheel, a tuning fork optical chopper, a dispersiveprism, a grating, an acousto optic modulator or a dielectric filter. 3.A device for determining a surface condition according to claim 1,characterised in that said wavelength modulation spectrometer comprisesa wavelength modulated laser.
 4. A system for determining and indicatinga surface condition, said system comprising a device for determining asurface condition according to claim 1, and comprising an indicatordevice for indicating the road surface condition determined by saiddevice for determining a surface condition.
 5. A method for determininga surface condition, said method using reflectance spectrometry forsensing the reflectance properties of a surface at at least onewavelength and using said reflectance properties to determine presenceof at least one of liquid water or ice, characterised in that saidreflectance spectrometry is wavelength modulation spectrometry, wherethe wavelength of the light is modulated at a frequency f, and theresulting amplitude modulation is detected at more than one multiple ofsaid frequency f.
 6. A method for determining a surface conditionaccording to claim 5, characterised in that light of said at least onewavelength is wavelength modulated before being reflected by saidsurface.
 7. A method for determining a surface condition according toclaim 5, characterised in that light of said at least one wavelength iswavelength modulated after being reflected by said surface.
 8. A methodfor determining a surface condition according to claim 5, characterisedin that light of said at least one wavelength also is intensitymodulated.
 9. A method for determining a surface condition according toclaim 5, characterised in that said method senses the reflectanceproperties of a surface at more than one wavelength.
 10. A method fordetermining a surface condition according to claim 9, characterised inthat said method uses the reflectance properties at the more than onewavelengths to determine the structural properties of the detectedliquid water or ice.